A group of 20 high school students partakes in an elite class to prepare for a college exam. Among them are the rebellious Kang I-na, her best friend Yoon Myong-heo, and the class clown Kang Hyun, who harbors an affection for I-na. Few days After the incidents of a student, Beom seeing a ghost haunt his paper and class teacher Hwang Chang-wook discovers scars on his hand, the class is interrupted by a TV showing of the top-ranking student, Min Hye-yeong, being trapped in a glass being slowly filled with water. A voice over the PA system announces they are part of a death game with questions that will be posed to the class; failure to answer them in time will lead to the student’s death. It also warns them and their two teachers, Hwang Chang-wook and Choi So-yeong, against leaving the school. The class fails to solve the first question to save Hye-yeong.Death Bell
Despite the warning, the class tries to leave, only to find that a teacher and hall monitor, Mr. Lee, was beaten to death, leaving only security Mr. Kim to guard them. The students split up and try to answer the questions while also attempting to find a way out. Several students die in brutal death game set up by a mysterious woman as the class solves all the puzzles late though later then notice that all the puzzle contains clue to a student, Kim Ji-won who died earlier that year due to mysterious circumstances. I-na discovers students are dying according to the ranking based on the last test the class then Beom, who earlier tries to strangle I-na and was put in a mental facility , escapes and attempts to kill I-na, but Hyun shields her from the attack and dies. Chang-wook then kills Beom. I-na discovers to her horror that Myong-heo has also been kidnapped by a mysterious woman. They succeed in solving the trap but the killer still kills her by dropping the girl from the roof and Chang-wook climbs the roof and strangles the woman to death.
A memorial is held for the students killed, then I-na is kidnapped unnoticed. One final question is posed: have the person who killed his daughter step forward and confess. The person in question, a now-demented Chang-wook, reveals himself and is killed by the mastermind, who turns out to be security, Mr. Kim. I-na is then freed by So-yeong.